Something isn't right, Baudenfj. One micron is 10^-6 meter Hg. Atmospheric pressure is 760mm Hz or 760,000 microns. So the first part looks right. But you aren't getting there or your mechanical gauge is way off.
Yes, my mechanical gauge most be off I do not even understand the units.. Pressure is force per unit area. If pressure is getting smaller the number must get smaller ! However on my mechanical gauge the numbers [inchers of Hg] are increasing as the pressure is getting samller!
Here is the missing link which I found on the internet
http://www.belljar.net/units.htm
The inch thing really got out of hand when manufacturers of rough pumps came on the scene. Rough pumps are arbitrarily defined as those used for: in-house vacuum systems; meat packing; impregnating lumber and transformer coils; making freeze dried coffee, tea or foods (got ya!). That is, any pump that hauls great loads of gas and vapor day-after-day to a modest vacuum level. These manufacturers noted that if atmospheric pressure was 29.92 inches Hg, they would be shooting for 0 inches. That would look bad in their brochures. So, they calmly inverted the scale. Atmospheric pressure is 0 inches Hg and the best possible vacuum is 29.92 inches Hg, they said. Which left the rest of us struggling with converting inches Hg to torr. (First, subtract the given inch pressure from 29.92 inches, then multiply the answer by 25.4.)
As for near 30" vacuum, yes, that is true that this is what you want to see. But on my mechanical gauge, the difference between -29 (possible) and -30 (impossible) is about .010 inches. Maybe .020 at most as I have not measured. But that is 25,000 microns of difference.
I agree it does not really tell us much in terms of microns... !
When my real gauge goes from 10,000 microns to 500, there is no discernible movement of the mechanical gauge. The mechanical gauge is not trustworthy at all, but sometimes it's all you have.